Yes the mantle is that rippling fleshy part. It's a bit hard to explain how they should appear. The best thing to do is just observe many maximas, both in person and as images, and you'll begin to see when one does not appear okay for whatever reason. There are books written about what you should look out for regarding clams, and I stongly recommend you either buy a couple or borrow them from a buddy. They're fun to read and will back up many things to look out for with pictures and the like. Also, there are many clams only forums that cater to the clam lover, where a boatload of information exists.
I recommend the book Giant Clams by Knop (although old and outdated in some respects, it's still a decent book). Also recommended highly is Reef Invertebrates by Calfo and Fenner which has an excellent clam section in it.